This invention relates generally to optical correlators, and, more particularly, to a phase-only optical correlation filter for use within an optical correlation system.
In recent years, the acceptance of optical correlation systems has greatly expanded because of their extreme usefulness in the processing of optical signals in, for example, the analysis of radar information or sonar information. More specifically, the optical correlation can effectively compare a pair of optical signals and by an analysis of intensity peaks determine information with respect to the optical signals. The Vander Lugt optical correlator, in particular, has proven to be an effective and valuable addition to the collection of optical processing systems. The Vander Lugt optical correlator has found applicability in such diverse applications as classifying diatoms and inspecting welding seams. It is an extremely powerful optical instrument because it can search an input beam for a desired pattern without any mechanical motion and provide a simple way to fabricate a matched filter. An excellent description of such a Vander Lugt optical correlator can be found in a paper published by A. Vander Lugt entitled "Signal Detection by Complex Spatial Filtering," IEEE Transactions on Information Theory, Volume IT-10, No. 2, April 1964, pages 139 through 145.
In a practical working optical correlation system it is desirable that the overall utilization of the laser light source be as efficient as possible. It is possible to measure the utilization of the laser light source by the ratio of light energy in the correlation spot in the output plane to the light energy at the input plane as discussed in greater detail by this inventor in an article entitled "Light Utilization in Optical Correlators," Applied Optics, Vol. 21, No. 24, 15 December 1982, pgs. 4511-4514. A high throughput of light is desireable in a practical working system in order to keep the laser power requirements as small as possible in order for such a correlation system to be usable in, for example, a portable or airborne system, and to achieve a high detector signal-to-scattered light (noise) ratio.
In the past, such as with the above-mentioned Vander Lugt optical correlator, the filter was formed holographically on a photosensitive plate such as, for example, a black and white holographic film such as #649F. The light throughput of the system is determined by the optical efficiency of the filter. Unfortunately, as pointed out by this inventor in the above-mentioned article entitled "Light Utilization in Optical Correlators," the efficiency of the matched filter utilized with the Vander Lugt correlator, even if made on a perfect medium, would only produce an efficiency of 44.4% for a simple rectangular function input. In a number of applications, where the use of a high power laser source is acceptable, such relatively low efficiency is permisible. However, in many of todays portable or airborn systems, in which a laser diode of low power is generally utilized as the light energy source, the efficiency of the matched filter becomes critical and therefore substantially limits the utilization of past optical correlators such as the above-mentioned Vander Lugt correlator.
In recent studies as set forth, for example, by Oppenheim and Lim, in an article entitled "The Importance of Phase in Signals," Proceedings of the IEEE, Volume 69, No. 5, May 1981, pages 529 through 541, it was found that phase information is considerably more important than amplitude information in preserving the visual intelligibility of a picture. Even more interesting, was the fact that when the phase information of one picture was combined with the amplitude information from an ensemble average of a group of pictures, the reconstructed picture was almost identical to the original picture. Similiar observations can be made about the Kinoform, which is a phase-only hologram, and reconstruction of atomic structure from x-ray diffraction data. Fourier synthesis of the structure from only the amplitude of the diffraction with zero phase does not reconstruct the correct structure, where as reconstruction from the phase data and unity amplitude does. Consequently, this inventor has determined that a phase-only filter incorporated within an optical correlation system could provide excellent correlation with a minimum of input power. Unfortunately, to date, producing a reliable phase-only filter for use within an optical correlation system has been unobtainable.